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  <div class="titlepage"><div><div><h1 style="clear: both">No-value policies</h1></div></div></div>
<p>In the previous section we have seen that it would be useful if calling member
function <code>.value()</code> on object of type <code>outcome&lt;T&gt;</code> that did not contain a value,
would cause an exception to be thrown according to some user-defined policy.</p>

<p>Let us consider <code>result&lt;T&gt;</code> first. It is an alias to <a href="../../reference/types/basic_result.html" class="api-reference"><code>basic_result&lt;T, E, NoValuePolicy&gt;</code></a>
,
where <code>E</code> is the type storing error information and defaulted to
<code>std::error_code</code>/<code>boost::system::error_code</code>, and <code>NoValuePolicy</code>
is a <em>no-value policy</em> defaulted to <a href="../../reference/aliases/default_policy.html" class="api-reference"><code>default_policy&lt;T, EC, EP&gt;</code></a>
.</p>

<p>The semantics of <code>basic_result::value()</code> are:</p>

<ol>
<li>Calls <code>NoValuePolicy::wide_value_check(*this)</code>.</li>
<li>Return a reference to the contained value. If no value is actually stored,
your program has entered undefined behaviour.</li>
</ol>

<p>Thus, the semantics of function <code>.value()</code> depend on the no-value policy. The
default policy (<code>policy::default_policy&lt;T, EC, void&gt;</code>) for <code>EC</code> of type
<code>std::error_code</code><sup class="footnote-ref" id="fnref:1"><a href="#fn:1">1</a></sup> does the following:</p>

<ul>
<li>If <code>r.has_value() == false</code> throws exception <code>std::system_error{r.error()}</code>,</li>
<li>otherwise no effect.</li>
</ul>

<div class="notices note" style="background: url('../../images/note.png') top left no-repeat padding-box padding-box;">
<div class="notices heading">note</div>
<div class="notices message"><p>Class templates <a href="../../reference/types/basic_result.html" class="api-reference"><code>basic_result&lt;T, E, NoValuePolicy&gt;</code></a>
 and <a href="../../reference/types/basic_outcome.html" class="api-reference"><code>basic_outcome&lt;T, EC, EP, NoValuePolicy&gt;</code></a></p>

<p>never use exceptions. Any exception-related logic is provided exclusively
through no-value policies.</p>
</div>
</div>


<p>When designing your own success-or-failure type using templates
<code>basic_result&lt;&gt;</code> or <code>basic_outcome&lt;&gt;</code> you have to decide what no-value policy
you want to use. Either create your own, or <a href="../../tutorial/essential/no-value/builtin.html">use one of the predefined policies</a>.</p>

<p>You can also use one of the two other predefined aliases for <code>basic_result&lt;&gt;</code>:</p>

<ul>
<li><a href="../../reference/aliases/unchecked.html" class="api-reference"><code>unchecked&lt;T, E = varies&gt;</code></a>
: it uses policy <a href="../../reference/policies/all_narrow.html" class="api-reference"><code>all_narrow</code></a>
, where any observation of a missing value or error is undefined behavior;</li>
<li><a href="../../reference/aliases/checked.html" class="api-reference"><code>checked&lt;T, E = varies&gt;</code></a>
:
it uses policy <a href="../../reference/policies/throw_bad_result_access.html" class="api-reference"><code>throw_bad_result_access&lt;EC&gt;</code></a>
, where any observation of a missing value or error throws <a href="../../reference/types/bad_result_access_with.html" class="api-reference"><code>bad_result_access_with&lt;EC&gt;</code></a>
 or <a href="../../reference/types/bad_result_access.html" class="api-reference"><code>bad_result_access</code></a>

respectively.</li>
</ul>
<div class="footnotes">

<hr />

<ol>
<li id="fn:1">Similar overloads exist for throwing <code>boost::system::system_error</code> when <code>EC</code> is <code>boost::system::error_code</code>.
 <a class="footnote-return" href="#fnref:1"><sup>[return]</sup></a></li>
</ol>
</div>



        </div><p><small>Last revised: February 08, 2019 at 22:18:08 UTC</small></p>
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